The present invention relates to passive optical networks and, more particularly, to a method of performing forward error correction initialization and auto-negotiation in passive optical networks.
Optical fiber links can best meet service providers' needs for high bandwidth combined with long reach for economical deployment. Fiber has taken over the long-haul network backbone and is emerging in the metropolitan area. Access networks, however, with their large number of endpoints, require a different network topology than SONET or DWDM used in long-haul and metro networks. A passive optical network (PON), including an optical line terminal (OLT) in the central office, optical network units (ONUs) at customer premises, shared fiber runs, and passive optical splitters, offers powerful advantages for broadband access networks.
For several decades Ethernet (IEEE 802.3) has been widely used in local area networks. More recently, Ethernet has been used with increasing frequency in metro and other wide area optical network applications, including passive optical networks.
Ethernet passive optical networks (EPONs), based on Gigabit Ethernet and the emerging IEEE 802.3ah protocol standard, offer the high capacity and low cost needed for broadband access wide area networks to be deployed widely and economically.
In the wide area network applications, signal attenuation and distortion become significant because of the distances involved and because the optical power splitters inherently attenuate signals. Forward error correction (FEC) is one method for reducing the bit error rate for a signal with an intrinsically low signal to noise ratio. FEC is a coding technique that uses additional, i.e. redundant symbols, as part of a transmission of a digital sequence through a physical channel. Because of the presence of redundancy, when errors corrupt the received signal, the receiver subsequently recognizes and corrects the errors without requesting data frame retransmission. Forward error correction in EPONs is described in co-pending PCT patent application IB02/01370 entitled, “Forward Error Correction Coding in Ethernet Networks”, assigned to Passave Networks Ltd., Tel Aviv Israel, and is incorporated herein by reference for all purposes as if fully set forth herein. PCT patent application IB02/01370 is referred to hereinafter as '370. As disclosed in '370, FEC coded data is received by a non-FEC OLT and/or ONU.
Presently, most of the OLTs and ONUs to be installed in new EPONs and upgrades of existing EPONs will support FEC. However, equipment already installed does not support FEC. Since FEC improves network performance when communications are limited by bit errors, it is desirable to employ FEC when available and required, despite the bit rate penalty, but it is not desirable to employ FEC when not required because of the bit rate penalty. Therefore, an automatic method is desirable in an EPON to acquire information from each ONU, when the ONU is installed, to determine if FEC is supported and required and implement FEC to decrease the bit error rate. If FEC is not available or not required, the communication between the OLT and an ONU uses ethernet frames without FEC coding.
There is thus a widely recognized need for, and it would be highly advantageous to have a method of performing forward error correction initialization and auto-negotiation in passive optical networks